The orbital data on this site are provided in a custom format that I call XTLE; it is an extension of the classic TLE (Two Line Element) format in use since the 1960s. TLE was an 80-byte format tailored for use with punch cards. XTLE relaxes this restriction and adds support for addtional metadata and for kinds of orbit than TLE cannot support.
The US astronautical community, and 18SPCS in particular, are evolving their format in a different direction, with the adoption of formats such as JSON that require more complicated parsing libraries. There are good reasons for this, but my own requirements led me to go for a format which I see as a minimal evolution of TLE, and in many cases back compatible with it.
There are two main flavours of XTLE, denoted flavour 1 and flavour 3 or XTLE1 and XTLE3. (Flavour 2 was an intermediate experiment that I used for a while but abandoned). Flavour 1 is used for most data and is largely back compatible with TLE. Flavour 3 is used for orbits whose central body is not Earth or where the eccentricity is greater than 1, or in cases where the catalog number is greater than 99999.
Both XTLE flavours have up to four lines of data, denoted line 0 to line 3.
XTLE1 Line 0 is provided to capture the additonal information in a JSON orbit record from Space-Track that doesn't fit in a regular TLE. It is mainly intended for inspection by the user rather than machine parsing.
| Start Byte | Field width | Field | Value | Comments |
|---|---|---|---|---|
| 1 | 1 | Line ID | 0 | Always '0' |
| 3 | 24 | Name | Satellite name | |
| 28 | 12 | Piece | COSPAR designation | |
| 41 | 1 | OType | Object type | X for Unknown/TBA, P for Payload, R for Rocket Body, D for Debris |
| 43 | 8 | Country | Country OrgCode | Code from Orgs file, e.g. CN, US |
| 52 | 8 | LSite | Launch Site | Code from Sites file, e.g. CC, V, PL |
| 61 | 12 | LDate | Launch Date | UTC Gregorian date, e.g. 2022 Apr 8 |
| 74 | 12 | DDate | Decay Date | Currently always "-" (null value) |
| 87 | 1 | Status | Status | 'O' for in orbit, 'R' for reentered; currently always "O" if set |
| 89 | 9 | Perigee | Perigee in km | SGP4 mean element conventional perigee height |
| 99 | 1 | spacer | 'x' | 'x' between peri and apo for readability |
| 101 | 9 | Apogee | Apogee in km | SGP4 mean element conventional apogee height |
Line 0 values are derived from the information in the 18SPCS JSON record, when available.
69 bytes: (back compatible with classic TLE)
| Start Byte | Field width | Field | Value | Comments |
|---|---|---|---|---|
| 1 | 1 | Line ID | 0 | Always '1' |
| 2 | 1 | Prefix | Catalog prefix | 'S' if absent; blank for classic TLE |
| 3 | 5 | Number | Catalog Number | 00001 to 99999; leading zeros optional |
| 8 | 1 | Classification | Secret or not? | Always 'U' for unclassified, if present |
| 10 | 8 | Piece | COSPAR designation | Optional. Packed version of Intl. Des. eg 68081J for 1968-081J. Pre 1963 designations converted to 18SPCS modern format if not blank. |
| 19 | 14 | Epoch | Packed Epoch Date | Example: 67335.2500 is 1967 Dec 1 0600. Line 3 gives time system, usually UTC |
| 34 | 10 | NDOT | Mean motion deriv | Units: rev per day squared |
| 45 | 8 | NDOT2 | Mean motion 2nd deriv | Units: rev per day cubed, packed format |
| 54 | 8 | BSTAR | BStar drag term | Packed format like NDOT2 |
| 63 | 1 | EPHTYPE | Ephemeris Type | Usually 0 |
| 65 | 4 | ELNO | Element number | Integer to ID the element set; may not be unique or reliable |
| 69 | 1 | PARITY | TLE Checksum | Checksum modulo 10, with minus signs given value 1 and all other non-digits 0 |
The GCAT catalog has various subcatalogs each with a different prefix. The S catalog (objects S00001 onwards) contains the same objects (more or less, modulo cross-tagging) as the USSF SATCAT. The A (auxiliary) catalog contains additional objects, some of which may have TLEs; so if bytes 2 to 9 of line 1 are 'A00035', the TLE refers to auxiliary catalog object A00035. This is the only respect in which my line 1 format may cause problems for parsers of standard TLEs.
The NDOT2 value has the form SME: sign character S = (+ or -), M (mantissa, 5 digits), E (exponent, sign and 1 digit), to be expanded as S x 0.M x 10**E. Example: +65942-4 maps to +0.65942E-4 = 0.00065942. The BSTAR value is formatted in the same way.
The ephemeris type was originally intended to encode whether Kozai or Brouwer mean motions were being used; however TLEs dating from 1962-1964 used Brouwer values without changing the ephemeris type value of 0. I am not aware of any valid TLEs in the SPCS archive with nonzero values of this parameter. I have manually edited the 1962-1964 TLEs that appear to use Brouwer mean motions, setting their ephemeris type to 2. (I recognized these TLEs on the assumption that the satellites involved had not suddenly jumped to a higher orbit for those epochs). My orbit software then does the right thing when it spots this value; I don't know if anyone else's software does.
The parity value may not be correct for TLEs whose origin is not NOR/SPTR. (In particular, I don't always correct it when I manually edit TLEs, origin = JCM).
69 bytes: (back compatible with classic TLE)
| Start Byte | Field width | Field | Value | Comments |
|---|---|---|---|---|
| 1 | 1 | Line ID | 0 | Always '2' |
| 2 | 1 | Prefix | Catalog prefix | 'S' if absent; blank for classic TLE |
| 3 | 5 | Number | Catalog Number | 00001 to 99999; leading zeros optional |
| 9 | 8 | Inc | Inclination, deg | Equator of frame, usually TEME equator of epoch |
| 18 | 8 | Node | RA of asc node, deg | In Frame, usually TEME coord system |
| 27 | 7 | Ecc | Eccentricity | Mantissa; prepend '0.' to determine value |
| 35 | 8 | AOP | Argument of perigee, deg | Measured from node to perigee |
| 44 | 8 | MeanAnom | Mean anomaly, deg | Measured from perigee |
| 53 | 11 | N | Mean motion, revs per day | |
| 64 | 5 | RevNo | Revolution number | Not reliable |
| 69 | 1 | Parity | TLE Checksum | See line 1 description |
The eccentricity must be in the range [0,1) and is formatted so that '0.5823981' is represented as '5823981', with the leading '0.' omitted.
For XTLE1 data the central body must be Earth.
XTLE1 cannot represent catalog numbers of 100000 or greater.
Line 3 is optional to allow back compatibility with classic TLE; however its presence is encouraged. Default values are provided in the table and these should be assumed to hold if line 3 is absent; the default flavour is 1.
| Start Byte | Field width | Field | Value | Comments |
|---|---|---|---|---|
| 1 | 1 | Line ID | 3 | Always '3' |
| 2 | 1 | Prefix | Catalog prefix | 'S' if absent; blank for classic TLE |
| 3 | 9 | Number | Catalog Number | Leading zeros optional; 5 or 9 digits |
| 13 | 1 | Flavour | 1 or 3 | Always '1' for XTLE1, '3' for XTLE3 |
| 15 | 6 | ORIGIN | Elset creator | See notes |
| 22 | 2 | Problem | Problem type | Error code; See notes |
| 25 | 4 | Eltype | Element model | Default SGP4; can be OSC (Osculating) |
| 30 | 4 | Frame | Coordinate system | Default TEME; sometimes ICRS or EC |
| 35 | 4 | TSYS | Time system | Default UTC; sometimes TDB |
| 40 | 30 | Primary | Central body | Name of central body, default Earth |
| 71 | 40 | Source | Source filename | Note to myself on filename or other source of raw data |
The ORIGIN values encode who originally made the TLE, and include among others:
| ORIGIN value | Meaning | Elset creator |
|---|---|---|
| NOR | NORAD | NORAD and its successors up to 2004, prior to the redistribution restrictions |
| SPTR | SpaceTrack | 18SPCS and its predecessors from 2005 onwards, via Space-Track.org, under Form 1 redistribution restrictions |
| UNK | Unknown | Source other than NORAD, unknown or too lazy to fill it in |
| JCM | Jonathan McDowell | TLEs I made up myself, sometimes actually based on evidence of some kind |
| CSS | Canadian Space Soc. | Hobbyist-created TLEs based on hobbyist observations; Molzcan, McCants, others; not necessarily actually from Canadian Space Soc, they were just the pioneers. |
| HOR | Horizons | JPL HORIZONS |
| TSK | T.S. Kelso | Celestrak Supplemental TLEs |
| PLU | Project Pluto | Bill Gray |
| DASO | Distant Artifical Satellites list | G. Williams, IAU MPC |
The PROBLEM values encode flags to warn you that the TLE set may be bogus or doubtful. There are also I and O flags, but so far I haven't used them much.
| PROBLEM value | Code | Meaning |
|---|---|---|
| I | INITIAL | Not a problem; flag to identify preferred TLE to use as initial orbit |
| O | OPERATIONAL | Not a problem; flag to identify preferred TLE to use as operational orbit |
| N | NULL | Orbit is null |
| S | SHAPE | Orbit is based on shape (apo x peri); Node/AOP/MA values not valid |
| G | GUESS | Orbit is a (possibly wild) guess and not based on observation |
| E | ERROR | Orbit is wrong in some way |
| EM | MISID | Orbit is cross-tagged with some other object |
| EW | WRONG | Just plain wrong |
| EI | INACCURATE | Orbit is valid but inaccurate |
| ED | DOUBTFUL | I am skeptical of the validity |
| EP | PREDICTED | Orbit is predicted rather than observed |
| EC | CORRUPTED | TLEs are corrupted in some way |
The central body names are provided in the Worlds table of the General Catalog of Artificial Space Objects, and corresponds to assumed masses in that table.
The layout of XTLE3 Line 1 is similar to classic TLE, except that an extra 4 bytes are provided for the catalog number, and the rest of the line is shifted correspondingly rightward. Also, 2 extra bytes are provided for the element number.
| Start Byte | Field width | Field | Value | Comments |
|---|---|---|---|---|
| 1 | 1 | Line ID | 0 | Always '1' |
| 2 | 1 | Prefix | Catalog prefix | 'S' if absent; blank for classic TLE |
| 3 | 9 | Number | Catalog Number | 000000001 to 999999999; leading zeros optional |
| 12 | 1 | Classification | Secret or not? | Always 'U' for unclassified, if present |
| 14 | 8 | Piece | COSPAR designation | Optional. Packed version of Intl. Des. eg 68081J for 1968-081J. Pre 1963 designations converted to 18SPCS modern format if not blank. |
| 23 | 14 | Epoch | Packed Epoch Date | Example: 67335.2500 is 1967 Dec 1 0600 UTC |
| 38 | 10 | NDOT | Mean motion deriv | Units: rev per day squared |
| 49 | 8 | NDOT2 | Mean motion 2nd deriv | Units: rev per day cubed, packed format |
| 58 | 8 | BSTAR | BStar drag term | Packed format like NDOT2 |
| 67 | 1 | EPHTYPE | Ephemeris Type | Usually 0 |
| 69 | 6 | ELNO | Element number | Integer to ID the element set; may not be unique or reliable |
| 75 | 1 | PARITY | TLE Checksum | Checksum modulo 10, with minus signs given value 1 and all other non-digits 0 |
The layout of XTLE3 Line 2 is generally similar to classic TLE, except that an extra 4 bytes are provided for the catalog number, and the rest of the line is shifted correspondingly rightward. Also, extra width is provided for several of the fields: the eccentricity is given with explicit decimal point and may be larger than 1; to support hyperbolic orbits, the mean anomaly can be in the range -9999 to +9999. The mean motion can be in exponential format (e.g. 9.9E7).
| Start Byte | Field width | Field | Value | Comments |
|---|---|---|---|---|
| 1 | 1 | Line ID | 0 | Always '2' |
| 2 | 1 | Prefix | Catalog prefix | 'S' if absent; blank for classic TLE |
| 3 | 9 | Number | Catalog Number | 00001 to 99999; leading zeros optional |
| 13 | 8 | Inc | Inclination, deg | Equator of epoch |
| 22 | 8 | Node | RA of asc node, deg | TEME coord system |
| 31 | 11 | Ecc | Eccentricity | Value 0 to 999; note that "0." is explicit. |
| 43 | 8 | AOP | Argument of perigee, deg | Measured from node to perigee |
| 52 | 10 | MeanAnom | Mean anomaly, deg | Measured from perigee. Value -9999 to +9999. |
| 63 | 20 | N | Mean motion, revs per day | |
| 84 | 6 | RevNo | Revolution number | Not reliable |
| 85 | 1 | Parity | TLE Checksum | See line 1 description |